Enzyme-containing granules are incorporated into products in several industries, including detergent, textile-processing, food (e.g., baking), animal feed, and fuel ethanol industries. Such granules may be prepared by a number of technologies, including fluidized bed spray coating, high sheer granulation, extrusion, spheronization, prilling, and spray drying.
Enzyme-containing granules of a small size (e.g., less than 300 micron diameter) are desirable in certain applications because small granules produce less dust and better protect enzymes against deactivating agents than do spray dried powders, while being easier to blend more homogeneously and inconspicuously with other powdered ingredients common in these industries. Such powders include surfactants and other detergent ingredients, buffers, salts, grain flours, starches, sugars, and/or inert diluents. A smaller granule has less tendency to segregate when blended into such powders and fine granular materials. Further, a given mass of smaller enzyme-containing granules will contain many more individual granules than the same mass of larger granules, and hence will provide greater homogeneity (less variation in net concentration) within a sample or aliquot of the powder into which it is blended, particularly for smaller sample sizes, e.g., less than about 50 to 100 grams of powder per aliquot. Granules with diameters in the range of 150 to 350 microns are advantageous because they are not so small that they produce large amounts of dust or are vulnerable to loss in enzyme potency and they are not so large that they blend poorly with typical powdered products.
It would be desirable to prepare such granules in a top-spray fluid bed coater because this technology can produce coated granules at a relatively low production cost and low equipment cost relative to its high productivity, i.e., mass of granules produced per unit time. With such a coating process, by judicious selection of the size distribution of the core particles, a resultant narrow and defined particle size distribution of final coated product can be achieved, which is advantageous for homogeneous quality and blending with powders.
A top-spray fluid bed coater is a coating vessel in which a bed of particles is suspended in a randomly circulating or churning bed produced by upward flow of air through a screen or retaining plate at the base of the vessel, and into which liquid coating solutions can be directed via spray nozzles inserted into the bed. Top-spray fluid bed coating is a well-established technology for producing coated enzyme-containing granules in a larger size range, i.e., where the majority of the granules have a diameter greater than 300 microns. However, with smaller granules, there is a strong tendency for the fluidized granules to agglomerate or stick together when sprayed with enzymes and other solutions, due to the presence of binders in the coating solutions. Agglomeration of fine powders is often employed deliberately to produce dry products, but such agglomerated powders are often undesirable because they typically have a broad particle size distribution, do not flow freely, and tend to form dusts or break down to fines when subjected to shear, impact, or other forces encountered in handling. Furthermore, coating of agglomerates is typically inefficient, since much of the coating material is incorporated into interstitial zones, rather than evenly coating the substrate particles.
There is a need for an improved method for production of small, uniformly coated, substantially discrete enzyme-containing granules in a top-spray fluid bed coating process.